Tschermakite

Formula: ☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2
Inosilicate (chain silicate)
Specific gravity: 3.15
Hardness: 5 to 6
Streak: Pale grey-green
Colour: Medium green to dark green to green-black to black, rarely brown
Environments:

Igneous environments
Metamorphic environments

Tschermakite occurs in eclogites and other ultramafic igneous rocks, and in medium- to high- grade metamorphic rocks, typically amphibolite (HOM). Kyanite and garnet are common associates (HOM, Mindat).

Alteration:

magnesium-rich tremolite, tschermakite and albite to pargasite and quartz
☐Ca2Mg5Si8O22(OH)2 + ☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2 + 2Na(AlSi3O8) ⇌ 2NaCa2(Mg4Al)(Si6Al2)O22(OH)2 + 8SiO2
(AM 92.4.491)

magnesium-rich tremolite, zoisite, corundum and H2O to tschermakite
3☐Ca2MgSi8O22(OH)2 + 2Ca2Al3[Si2O7][SiO4]O(OH) + 7Al2O3 + H2O ⇌ 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2
(AM 76.990)

Common impurities: Ti,Mn,Na,K,H2O,Fe

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